Hard disk drives (hereinafter referred to as “HDD”) in widespread use have a structure in which one or a plurality of magnetic disks arranged concentrically are driven by a spindle motor. Reading and writing of data in the HDD is carried out by a magnetic head disposed so as to oppose the magnetic disk. The magnetic head is driven by an actuator. As the actuator, a swing operation type voice coil motor (hereinafter referred to as “VCM”) is used in general.
With reference to FIG. 7, a typical configuration and operation of the VCM will now be explained. As depicted, the VCM comprises a pair of yokes 15 disposed so as to oppose each other vertically, a permanent magnet member 10 disposed between the pair of yokes 15 and bonded to the lower yoke 15; and a head carriage 17, disposed so as to be rotatable about a shaft 18, having a fan-shaped coil 16 arranged in a magnetic gap formed between the upper yoke 15 and permanent magnet member 10.
When a predetermined current flows through the coil 16 in this VCM, a driving force occurs in any of directions of arrows A in the coil 16 in conformity to Fleming's left hand rule, whereby the head carriage 17 rotates about the shaft 18 in any of directions of arrows B. Because of such an action of the VCM, a magnetic head 19 mounted to the leading end part of the head carriage 17 moves in any of directions of arrows C which is opposite from the driving force generated in the coil 16. As a consequence, the magnetic head 19 can be positioned with respect to a magnetic disk 20.
Employed as the permanent magnet member 10 used in the VCM is an R-T-B type rare-earth permanent magnet material (wherein R is at least one kind of rare-earth element including Y, whereas T is at least one kind of transition metal element including Fe or Fe and Co as an essential ingredient), since it yields excellent magnetic characteristics. This rare-earth permanent magnetic material exhibits a low resistance to corrosion, since R and Fe, which are main constitutional elements thereof, are quite easy to oxidize. Therefore, when using this material as the permanent magnet member 10, the surface of a magnet body consisting of the permanent magnet material is usually coated with a corrosion-resistant film. For such a corrosion-resistant film, Ni or Ni alloy plating, which is excellent in resistance to corrosion, reliability, cleanliness, etc., is often employed. As the yokes 15, on the other hand, a silicon steel plate whose surface is provided with electroless Ni plating is often used.
Meanwhile, for responding to the recent speedup in information processing, HDDs employed as data storage means are required to be driven at a higher speed. This makes it necessary for the magnetic disk 20 to rotate at a high speed, which requires the VCM to be driven fast correspondingly thereto. In the conventional VCM, the permanent magnet member is used in the state secured to the yoke 15 as mentioned above typically by way of an adhesive layer. In order for the VCM to fully secure the durability at the time of high-speed driving, it is desirable that the permanent magnet member 10 and the yoke 15 be bonded firmly to each other.
Japanese Patent Application Laid-Open No. 2002-158105 discloses a method in which the surface of the Ni plating film in the permanent magnet member is phosphated with a processing solution having a specific composition. In this method, a phosphate coating having a desirable thickness is formed on the Ni plating film. The resulting magnet can effectively eliminate the poor hardening of an adhesive which is not reactive on the Ni plating film. This can reduce fluctuations in the bonding strength due to the adhesive, and can attain a bonding strength greater than that conventionally available. As a result, a higher efficiency can be achieved in the bonding operation.